1301, when it spread over a considerable portion of Italy, causing
great loss among the war horses of Rome and the surrounding district.
Later, in 1648, an epizootic of this disease visited Germany and spread
to other parts of Europe. In 1711, under the name of "epidemica
equorum," it followed the tracks of the great armies all over Europe,
causing immense losses among the horses, while rinderpest was scourging
the cattle of the same regions. The two diseases were confounded with
each other, and were, by the scientists of the day, supposed to be
allied to the typhus, which was a plague to the human race at the same
time. We find the first advent of this disease to the British Islands in
an epizootic among the horses of London and the southern counties of
England in 1732, which is described by Gibson. In 1758 Robert Whytt
recounts the devastation of the horses of the north of Scotland from the
same trouble. Throughout the eighteenth century a number of epizootics
occurred in Hanover and other portions of Germany and in France, which
were renewed early in the present century, with complications of the
intestinal tract, which obtained for it its name of gastro-enteritis. In
1766 it first attacked the horses in North America, but is not described
as again occurring in a severe form until 1870-1872, when it spread over
the entire country, from Canada south to Ohio, and then eastward to the
Atlantic and westward to California. It is now a permanent disease in
our large cities, selecting for the continuance of its virulence young
or especially susceptible horses which pass through the large and
ill-ventilated and uncleaned stables of dealers, and assumes from time
to time an enzootic form, when from some reason its virulence increases.
It assumes this form also when, from reasons of rural economy and
commerce, large numbers of young and more susceptible animals are
exposed to its contagion.
_Etiology._--The experiments of Dieckerhoff many years ago proved that
the disease may be transmitted to healthy animals by intravenous
injection of warm blood from affected horses.
Further investigations revealed the fact that blood from affected
horses, even when passed through porcelain filters, may transmit the
disease, thereby proving that the causative agent belongs to the
so-called filterable viruses. This has been further substantiated by
Gaffky, who showed in his recent experiments that the disease may be
transmitted with defibrinated as well as with filtered blood, in which
cases the typical form of influenza developed in inoculated animals in
from five to six days. These findings were also substantiated by Basset.
Further observations have also proved that apparently recovered animals
may harbor the infection for a long time and still be capable of
transmitting the disease. Such virus carriers are no doubt responsible
for numerous outbreaks of this disease when, in a locality free from
the disease, it certainly appears after the introduction of an
apparently healthy animal.
As one attack is usually self-protective, numbers of old horses, having
had an earlier attack, are not capable of contracting it again; but,
aside from this, young horses, especially those about four or five years
of age, are much more predisposed to be attacked, while the older ones,
even if they have not had the disease, are less liable to it. Again, the
former age is that in which the horse is brought from the farm, where it
has been free from the risk of exposure, and is sold to pass through the
stables of the country taverns, the dirty, infected railway cars, and
the foul stockyards and damp stables of dealers in our large cities.
Overfed, fat, young horses which have just come through the sales
stables are much more susceptible to contagion than the same horses are
after a few months of steady work.
Pilger, in 1805, was the first to recognize infection as the direct
cause of the disease. Roll and others studied the contagiousness of
influenza, and, finding it so much more virulent and permanent in old
stables than elsewhere, classed it as a "stall miasm." The contagion
will remain in the straw bedding and droppings of the animal and in the
feed in an infected stable for a considerable time and if these are
removed to other localities it may be carried in them. It may be carried
in the clothing of those who have been in attendance on horses suffering
from the disease. The drinking water in troughs and even running water
may hold the virus and be a means of its communication to other animals,
even at a distance.
The studies of Dieckerhoff, in 1881, in regard to the contagion of
influenza were especially interesting. He found that during a local
enzootic, produced by the introduction of infected horses into an
extensive stable otherwise perfectly healthy, the infection took place
in what at first seemed to be a most irregular manner, but which was
shown later to be dependent on the ventilation and currents of air
through the various buildings. His experiments showed that the virus of
influenza is excessively diffusible, and that it will spread rapidly to
the roof of a building and pass by the apertures of ventilation to
others in the neighborhood. The writer has seen cases that have appeared
to spread through a brick wall and attack animals on the opposite side
before others even in the same stable were affected. Brick walls, old
woodwork, and the dirt which is too frequently left about the feed boxes
of a horse stall will hold the contagion for several days, if not weeks,
and communicate it to susceptible animals when placed in the same
locality. On two successive mornings a 4-year-old colt belonging to the
writer stood for about 10 minutes at the open door, fully 40 feet from
the stalls, of a stable in which two cases of influenza had broken out
the day before: in six days the colt developed the disease. On the
morning when the trouble in the colt was recognized it stood in an
infirmary with a dozen horses that were being treated for various
diseases, but was immediately isolated; within one week two-thirds of
the other horses had contracted the disease.
_Symptoms._--After the exposure of a susceptible horse to infection a
period of incubation of from four to seven days elapses, during which
the animal seems in perfect health, before any symptom is visible. When
the symptoms of influenza develop they may be intense, or so moderate as
to occasion but little alarm, but the latter condition frequently
exposes the animal to use and to the danger of the exciting causes of
complications which would not have happened had it been left quietly in
its stall in place of being worked or driven out to show to prospective
purchasers. The disease may run a simple course as a specific fever,
with alterations only of the blood, or at any period it may become
complicated by local inflammatory troubles, the gravity of which is
augmented by developing in an animal with an impoverished blood, an
already irritated, rapid circulation, and defective nutritive and
reparative functions.
The first symptoms are those of a rapidly developing fever, which
becomes intense within a very short period. The animal becomes dejected
and inattentive to surrounding objects; stands with its head down, and
not back on the halter as in serious lung diseases. In the flanks, the
muscles of the croup and of the shoulders, or of the entire body it has
chills lasting from 15 to 30 minutes, and frequently a grinding of the
teeth which warns one that a severe attack may be expected. The hairs
become dry and rough and stand on end. The body temperature increases to
104 deg., 104.5 deg., and 105 deg. F., or even in severe cases to 107 deg. F., within
the first twelve or eighteen hours. The horse becomes stupid, stands
immobile with its head hanging, the ears listless, and it pays but
little attention to the surrounding attendants or the crack of a whip.
The stupor becomes rapidly more marked, the eyes become puffy and
swollen with excessive lacrimation, so that the tears run from the
internal canthus of the eye over the cheeks and may blister the skin in
their course. The respiration becomes accelerated to 25 or 30 in a
minute, and the pulse is quickened to 70, 80, or even 100, moderate in
volume and in force. There is great depression of muscular force; the
animal stands limp, as if excessively fatigued. There is diminution, or
in some cases total loss, of sensibility of the skin, so that it may be
pricked or handled without attracting the attention of the animal. On
movement, the horse staggers and shows a want of coordination of all the
muscles of its limbs. The senses of hearing, sight, and taste are
diminished, if not entirely destroyed. The visible mucous membranes (as
the conjunctiva), from which it received the name pinkeye, and the
mouth, and the natural openings become of a deep saffron, ocher, or
violet-red color. This latter is especially noticeable on the rim of the
gums and is a condition not found in any other disease, so that it is an
almost diagnostic symptom. In some outbreaks there is much more swelling
of the lids and weeping from the eyes than in others. If the animal is
bled at this period the blood is found more coagulable than normal, but
at a later period it becomes of a dark color and less coagulable. There
is great diminution or total loss of appetite, with an excessive thirst,
but in many cases cold-blooded horses may retain a certain amount of
appetite, eating slowly at hay, oats, or other feed. There is some
irritation of the mucous membrane of the respiratory tract, as shown by
discharge of mucus from the nose, and by cough. Pregnant mares are
liable to abort.
We have, following the fever, a tumefaction, or edema, of the
subcutaneous tissues at the fetlocks, of the under surface of the belly,
and of the sheath of the penis, which may be excessive. The infiltration
is noninflammatory in character and produces an insensibility of the
skin like the excessive stocking which we see in debilitated animals
after exposure to cold. In ordinary cases the temperature has reached
its maximum of 105 deg. or 106 deg. F. in from 24 to 48 hours from the origin of
the fever. It remains stationary for a period of from 3 to 4 days
without so much variation between morning and evening temperature as we
have in pneumonia or other serious diseases of the lungs. At the
termination of the specific course of the disease, which is generally
from 6 to 10 days, the fever abates, the swelling of the legs and under
surface of belly diminishes, the appetite returns, the strength is
rapidly regained, the mucous membranes lose their yellowish color, which
they attain so rapidly at the commencement of the disease, and the
animal convalesces promptly to its ordinary good condition and health,
and rapidly regains the large amount of weight which it lost in the
early part of the disease, a loss which frequently reaches 30, 50, or
even 75 pounds each 24 hours. For the first three days of the high
temperature there is a great tendency to constipation, which should be
avoided if possible by the use of the means recommended below, for, if
it has been marked, it may be followed by a troublesome diarrhea.
_Terminations._--The terminations of simple influenza may be death by
extreme fever, with failure of the heart's action; from excessive coma,
due generally to a rapid congestion of the brain; to the poisonous
effects of the debris of the disintegrated blood corpuscles and the
toxin of the disease; to an asphyxia, following congestion of the lungs;
or the disease terminates by subsidence of the fever, return of the
appetite and nutritive functions of the organs, and rapid
convalescence; or, in an unfortunately large number of cases, the
course of the disease is complicated by local inflammatory troubles,
whose gravity is greater in influenza than it is when they occur as
sporadic diseases.
_Complications._--The complications are congestions, followed by
inflammatory phenomena in the various organs of the body, but they are
most commonly located in the intestines, lungs, brain, or vascular
laminae of the feet. Atmospheric influence or other surrounding
influences of unknown quality seem to be an important factor in the
determination of the local lesions. At certain seasons of the year, and
in certain epizootics, we find 40 and 50 per cent or even a greater
percentage of the cases rendered more serious by complication of the
intestines; at other seasons of the year, or in other epizootics, we
find the same percentage of cases complicated by inflammation of the
lungs, while at the same time a small percentage of them are complicated
by troubles of the other organs; inflammatory changes of the brain, of
the laminae, more rarely commence in epizootic form, but are to be found
in a certain small percentage of cases in all epizootics.
Exciting causes are important factors in complicating individual cases
of influenza, or in localizing special lesions, during either enzootics
or epizootics. These exciting or determining causes act much as they
would in sporadic inflammatory diseases, but in this case we find the
animal much more susceptible and predisposed to be acted upon than
ordinary healthy animals. With a temperature already elevated, with the
heart's action driving the blood in increased quantity into the
distended blood vessels, which become dilated and lose their
contractility, with a congestion of all the vascular organs already
established, it takes but little additional irritation to carry the
congestion one step further and produce inflammation.
_Complication of the intestines._--When any cause acts as an irritant to
the intestinal tract during the course of this specific fever it may
produce inflammation of the organs belonging to it. This cause may be
constipation, which can find relief only in a congestion which offers to
increase the function of the glands and relieve the inertia caused by a
temporary cessation of activity; or irritant medicines, especially any
increased use of antimony, turpentine, or the more active remedies; the
taking of indigestible feed, or of feed in too great quantities, or that
has been altered in any way by fungus or other injurious alterations;
the swallowing of too cold water; or any other irritant may cause
congestion. This complication is ushered in by colics. The animal paws
with the fore feet and evinces a great sensibility of the belly; it
looks with the head from side to side, and may lie down and get up, not
with violence, but with care for itself, perfectly protecting the
surface of the belly from any violence. At first we find a decided
constipation; the droppings if passed are small and hard, coated with a
viscous varnish or even with false membranes. In from 36 to 40 hours the
constipation is followed by diarrhea. The alimentary discharge becomes
mixed with a sero-mucous exudation, which is followed by a certain
amount of suppurative matter. The animal becomes rapidly exhausted and
unstable, staggers on movement, losing the little appetite which may
have remained, and has exacerbations of fever. The pulse becomes softer
and weaker, the respiration becomes gradually more rapid, the
temperature is about 1 deg. to 1.5 deg. F. higher. If a fatal result is not
produced by the extensive diarrhea the discharge is arrested in from 5
to 10 days and a rapid recovery takes place.
_Complication of the lungs._--If at any time during the course of the
fever the animal is exposed to cold or drafts of air, or in any other
way to the causes of repercussion, the lungs may become affected. In the
majority of cases, however, after three, four, or five days of the
fever, congestion of the lungs commences without any exposure or
apparent exciting cause. Unless this congestion of the lungs is soon
relieved it is followed by an inflammation constituting pneumonia. This
pneumonia, while it is in its essence the same, differs from an ordinary
pneumonia at the commencement by an insidious course. The animal
commences to breathe heavily, which is distinctly visible in the heaving
of the flanks, the dilatation of the nostrils, and frequently in the
swaying movement of the unsteady body. The respirations increase in
number, what little appetite remained is lost, the temperature increases
from 1 deg. to 2 deg., the pulse becomes more rapid, and at times, for a short
period, more tense and full, but the previous poisoning of the specific
disease has so weakened the tissues that it never becomes the
characteristic full, tense pulse of a simple pneumonia.
On percussion of the chest dullness is found over the inflamed areas; on
auscultation at the base of the neck over the trachea a tubular murmur
is heard. The crepitant rales and tubular murmurs of pneumonia are heard
on the sides of the chest if the pneumonia is peripheral, but in
pneumonia complicating influenza the inflamed portions are frequently
disseminated in islands of variable size and are sometimes deep-seated,
in which case the characteristic auscultory symptoms are sometimes
wanting. From this time on the symptoms of the animal are those of an
ordinary grave pneumonia, rendered more severe by occurring in a
debilitated animal. The cough is at first hacky and aborted; later, more
full and moist. There is discharge from the nostrils, which may be
mucopurulent, purulent, or hemorrhagic. As in simple pneumonia, in the
outset this discharge may be "rusty," owing to capillary hemorrhages. We
find that the blood is thoroughly mixed with the matter, staining it
evenly, instead of being mixed with it in the form of clots. At the
commencement of the complication the animal may be subject to chills,
which may again occur in the course of the disease, in which case, if
severe, an unfavorable termination by gangrene may be looked for. If
gangrene occurs it is shown by preliminary chills, a rapid elevation of
temperature, a tumultuous heart, a flaky discharge from the nostrils,
and a fetid breath; the symptoms are identical with those which occur in
gangrene complicating other diseases.
_Complication of the brain._--At any time during the course of the
disease congestion of the brain may occur; at an early period if the
fever has been intense from the outset, but in ordinary cases more
frequently after three or four days. The animal, which has been stupid
and immobile, becomes suddenly restless, walks forward in the stall
until it fastens its head in the corner. If in a box stall and it
becomes displaced from its position, it follows the wall with the nose
and eyes, rubbing it along until it reaches the corner and again fastens
itself. It may become more violent and rear and plunge. If disturbed by
the entrance of the attendant or any loud noise or bright light, it will
stamp with its fore feet and strike with its hind feet, but is not
definite in fixing the object which it is resisting, which is a
diagnostic point between meningitis and rabies and which renders the
animal with the former disease less dangerous to handle. If fastened by
a rope to a stake or post, the animal will wander in a circle at the end
of the rope. It wanders almost invariably in one direction. The pupils
may be dilated or contracted, or we may find one condition in one eye
and the opposite in the other.
The period of excitement is followed by one of profound coma, in which
the animal is immobile, the head hanging and placed against the corner
of the stall, the body limp, and the motion, if demanded of the animal,
unsteady. Little or no attention is paid to the surrounding noises, the
crack of a whip, or even a blow on the surface of the body. The
respiration becomes slower, the pulsations are diminished, the coma
lasts for variable time, to be followed by excesses of violence, after
which the two alternate, but if severe the period of coma becomes longer
and longer until the animal dies of spasms of the lungs or of heart
failure. It may die from injuries which occur in the ungovernable
attacks of violence.
_Complication of the feet._--The feet are the organs which are next in
frequency predisposed to congestion. This congestion takes place in the
laminae (podophyllous structures) of the feet. The stupefied animal is
roused from its condition by excessive pain in the feet and assumes the
position of a foundered horse; that is, if the fore feet alone are
affected, they are carried forward until they rest on the heels; and if
the hind feet are affected, all the feet are carried forward, resting on
their heels, the hind ones as near the center of gravity as possible. In
some cases the stupor of the animal is so great that the pain is not
felt, and little or no change of the position of the animal is
noticeable. The foot is found hot to the touch, and after a given time
the depressed convex sole of typical founder is recognized.
_Pleurisy._--This is a rare complication, but when it does occur it is
ushered in by the usual symptoms of depression, rapid pulse, small
respiration, elevation of the temperature, subcutaneous edema of the
legs and under surface of the belly, and we find a line of dullness on
either side of the chest and an abscess of respiratory murmur at the
lower part. If it is severe, there may be an effusion filling one-fourth
to one-third of the thoracic cavity in from 36 to 48 hours.
_Pericarditis_ is an occasional complication of influenza. It is ushered
in by chills, elevation of the temperature; the pulse becomes rapid,
thready, and imperceptible. The heart murmurs become indistinct or can
not be heard. A venous pulse is seen on the line of the jugular veins
along the neck. Respiration becomes more difficult and rapid. If the
animal is moved the symptoms become more marked or it may drop suddenly
dead from heart failure.
_Peritonitis_, or inflammation of the membranes lining the belly and
covering the organs contained in it, sometimes takes place. The general
symptoms are similar to those of a commencing pericarditis. The local
symptoms are those of pain, especially to pressure on side of the flanks
and belly, distention of the latter, and sometimes the formation of
flatus, or gas, and constipation.
Other occasional complications are nephritis, hepatitis, inflammation of
the flexor tendons and rupture of them, and abscesses.
_Diagnosis._--The diagnosis of influenza is based upon continued fever,
with great depression and symptoms of stupor and coma; the rapidly
developing, dark-saffron, ocher, yellowish discoloration of the mucous
membranes, swelling of the legs and soft tissues of the genitals. When
these symptoms have become manifested the diagnosis of a local
complication is based upon the same symptoms that are produced in the
local diseases from other causes, but in influenza the local symptoms
are frequently masked or even entirely hidden by the intense stupor of
the animal, which renders it insensible to pain. The evidence of colic
and congestion, which is followed by diarrhea, indicates enteritis. The
rapid breathing or difficulty of respiration points to a complication of
the lungs, but, as we have seen in the study of the symptoms, the local
evidences of lung lesions are frequently hidden. Again, we have seen
that inflammation of the feet, or founder, complicating influenza is
frequently not shown on account of the insensibility to pain on the part
of the animal, which indicates the importance of running the hand daily
over the hoofs to detect any sudden elevation of temperature on their
surface.